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oil filter bypass pressure issues -long-
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Posted by Dean (more from Dean) on Sat, 19 Apr 2003 09:48:47 Share Post by Email
In Reply to: PF20 vs PF13?, Greg Netzner, Fri, 18 Apr 2003 15:34:10
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There are two relief valves. One is in the pump it self and it limits the pressure that the pump can deliver. The other is in the filter itself and its limits the pressure across the fitler element. This pressure across the element can increase if the fitler has become fouled with crap. This pressure also goes up with cold oil. If you are not neglecting oil and filter changes the first will not be an issue. With very cold weather, the filter bypass can start to open. Higher rpm's with a cold engine can contribute to this. The valve limits the pressure across the filter element. So if the valve limited the pressure across the filter media to 15 PSI, the flow through the media at 15 PSI still occurs. Only part of the flow in the filter bypasses the media. So a lot of fitlering occurs. The pressure relief valve in the oil pump does not know or care that the fitler is bypassing. So if the filter starts to bypass some oil flow, the overall pump flows and pressures are not effected by that.

The pressure relief of the filter protects the filter media from pressures high enough to to physcally collapse the media. It also limits the pressure drop that the bearings and other things see to, in this case, as 15 PSI drop contribution from the filter. Note that there will be pressure drops from oil passages with cold oil.

So when a bypass relief valve flows in a filter, there is still the 15PSI or so pressure across the filter media. There is really no risk of trapped solids suddenly getting back into the flow. And whatever oil bypassed the filter media is simply as clean as the oil in the sump which has been filtered over and over again. Really no problems there. Many folks seem to be freaked out about the prospect of the filter relief valve ever going into flow, but I can't see any problems with this at all. Its a limited portion of the flow and only occurs, if at all, during extreme cold. The duration is limited as the oil thins out rapidly a the block warms up.

Now does any of this matter? With a [0|5|10]W[30|40] fully synthetic oil, such as Mobil 1 or other true synthetics, I would expect that there is never any cause for the filter element bypass filter to flow any oil. It is a fail safe mechanism mostly. Like the fuses in a system that are good to have, but they don't function very often or at all. So with a good oil and grade, whether a filter bypasses at 15 or 18 PSI is not really an issue. And in severe cold, which few of us ever see, one would hope that the engine is warmed up with care and not raced.

But standard are created and persist. Diesel engine oil filters have higher BP relief pressures. Oils have typically been 15W40 or 15W50, and dyno until recently. So the relief pressure are higher and partly out of concern that a diesel engine takes a long time to warm up. So the time duration of fitler bypass could be quite extended. So if you look for an alternative filter for a diesel, the official selections are quite small because of the BP pressures. But if one is using a 5W40 synthetic such as Delvac 1, then using a filter spec'd for a gas engine would not involve any risks. So when you make that leap, suddenly there are hundreds of alternatives to evaluate. The diesel BP pressures are spec'd for a worst case, 15W40 dyno oil with cold starts where the oil does not pump or filter very well.

Thats the way that I see it. If others want to adjust my viewpoint, I am always wanting to learn more.

posted by 66.139.120...
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